A semiconductor element for power control may also be called a power semiconductor element, and a semiconductor device for power control in which this is used may also be called a power module. The power control semiconductor device has a function of controlling a relatively large current, and is used for a control device for a motor or the like. Power control semiconductor devices currently used include a molding-sealed type in which semiconductor elements are sealed with a thermosetting resin, such as an epoxy resin, and a gel-sealed type in which semiconductor elements are sealed with a gel resin. In particular, the molding-sealed type semiconductor device is widely used for controlling an air-conditioner or the like because of its compactness, excellent reliability and easy handling. In recent years, it is also used for power control for a vehicle in which motor driving is performed.
The molding-sealed type semiconductor device is assembled by first performing die-bonding to adhere semiconductor elements to a frame, and then performing interconnection directly to leads or to corresponding lead parts with wires. The semiconductor elements are obtained by cutting a thin-plate-like crystal wafer into rectangular and separate pieces, and thus they are also called semiconductor chips. After performing interconnection, molding sealing is carried out with a sealing resin, such as an epoxy resin, by a technique such as transfer molding or potting. In the above-described semiconductor device, thermal stress resulting from the difference in thermal expansion coefficient between the semiconductor chips and the sealing resin (molding resin) or shrinkage on curing of the sealing resin is produced in the semiconductor chips. Since intermittent current application to the semiconductor elements is repeated in the operation of the semiconductor device, the semiconductor elements repeat the temperature cycle of temperature rise and temperature drop every time electric current is applied. Therefore, if a defect is brought about by the above-described thermal stress in the adhesion interface between the sealing resin and the semiconductor chips to cause peeling or microcracking of resin, reliability might be degraded due to degradation in withstand voltage, variation in characteristics of the elements, or the like. Even if peeling is not brought about, deformation of interconnection lines or breakage of the semiconductor chips may occur. Since the above-described thermal stress is maximized at the end of each semiconductor chip, peeling of resin originates in many cases from the end of each semiconductor chip.
In a conventional semiconductor device, a polyamide resin is applied to the whole surface of an assembly associated with interconnection, including a heat sink, semiconductor chips, an electrode block, and the like to thereby increase adhesion with a sealing resin (see e.g., Patent Document 1).
Similarly, the surface of an assembly (housing component) is covered thinly with a polyimide- or polyamide imide-based covering resin, and a sealing resin is charged thereon and cured so as to improve the resistance to temperature cycle and moisture resistance (see e.g., Patent Document 2).